The keyless entry system which locks/unlocks vehicle door by a remote control operation has become popular, and the design of vehicles has become diversified. There are increasing demands for a compact antenna device that can be installed easily on such sorts of vehicles.
A conventional antenna device for vehicles is described in the following, referring to FIG. 7 and FIG. 8.
FIG. 7 is a perspective view showing the key part of a conventional vehicle antenna device. In FIG. 7, core 1 is made of a ferrite material. Bobbin 2 has a blank at the central portion, and the holding portion is made of a dielectric resin. Outer surface of bobbin 2 is wound around with a linear conductor to form coil 3. The central portion of bobbin 2 is penetrated by core 1.
Case 4 is for housing bobbin 2 and other items. Case 4 is provided with lead terminals 5, 6 and relay elements 13, 14, which are insert-molded.
Relay element 13 is electrically connected with lead 3A, which being one of the two ends of coil 3, by soldering or the like means; while lead terminal 5 is likewise connected with lead 3B, which being the other end of coil 3.
Relay element 13 is electrically connected also with lead 9A, which being one of the two ends of capacitor 9, while the other end of capacitor 9, or lead 9B, is connected with relay element 14.
Furthermore, lead 11A, which being one of the two ends of resistor 11, is electrically connected with relay element 14, while the other end, or lead 11B, is connected with lead terminal 6.
The space within case 4 excluding coil 3, core 1, bobbin 2, capacitor 9, resistor 11, relay elements 13 and 14, and lead terminals 5 and 6 is filled with flexible dielectric resin 12.
As shown in a circuit diagram, FIG. 8, coil 3, capacitor 9 and resistor 11 are connected in series. Both ends of which serially connected line are connected with lead terminals 5 and 6, respectively. Vehicle antenna device 15 has such configuration.
Lead terminals 5 and 6 are electrically coupled with a control circuit (not shown) built in a vehicle body. The control circuit and vehicle antenna device 15 integrally constitute a serial resonance circuit.
Japanese Patent Laid-Open Application No. 2001-345615 represents a known prior art information relevant to the present invention.
In the above-described conventional vehicle antenna device 15, since core 1 has to be inserted into the blank central portion of bobbin 2 a certain clearance needs to be provided between bobbin 2 and core 1. Furthermore, body of bobbin 2 needs to have a certain specific thickness to ensure a high physical strength enough to withstand a possible deformation caused when coil 3 is formed. As the result, the overall size of vehicle antenna device 15 naturally becomes bulky.
The conventional vehicle antenna device and a vehicle antenna system and a communication system containing the antenna device are described below referring to FIG. 9.
FIG. 9 is a drawing used to show the concept of a conventional communication system. In FIG. 9, vehicle antenna device 15 transmits/receives electromagnetic waves for communication with a third party; fixing portion 16 is for installing a mirror close to the door at driver's seat, and at passenger seat. Movable portion 17 is for holding a mirror, and foldable.
Vehicle antenna device 15 is housed in movable portion 17; vehicle antenna device 15 and movable portion 17 integrally form vehicle antenna system 18.
Card 19 is proprietary to a vehicle equipped with vehicle antenna system 18 for communication. A communication system is structured by a combination of card 19, vehicle antenna system 18 and a control circuit (not shown) connected to vehicle antenna system 18.
Under the above setup, the control circuit regularly outputs a signal to card 19 held by a vehicle driver, requesting to respond with the ID code. Vehicle antenna device 15 transmits the signal after converting it into electromagnetic wave of a certain specific resonance frequency.
When a vehicle driver holding card 19 makes access to be close to the vehicle and card 19 receives the electromagnetic wave, card 19 transmits ID code signal after converting it into electromagnetic wave.
After that, when the electromagnetic wave is received by vehicle antenna device 15 and the incoming ID code is recognized to be true by the control circuit, the vehicle door is automatically unlocked. In a case when it is not recognized to be true, or failed to receive the ID code sent from a vehicle driver going away from the vehicle, the door is automatically locked.
Japanese Patent Laid-Open Application No. 2002-30844 is a known prior art information relevant to the present invention.
Under the above-described conventional vehicle antenna device 15 and a communication system which contains vehicle antenna device 15, however, the directivity of antenna is dislocated when movable portion 17 holding a mirror is folded by a vehicle driver. Vehicle antenna device 15 built in movable portion 17 is moved to a different orientation. As a result, there has been a problem that a reliable remote control operation is not ensured even when a driver is within a certain distance range from the vehicle.